Recent years have seen an introduction of teleconferencing systems to establish connections between plural points via the IP (Internet Protocol). A conventional conference system uses a speech coding technique including G. 726 to establish connections between each of points on one-to-one basis.
Providing a monophonic acoustic signal based on a monophonic speech coding technique including the G. 726 described in Non-Patent Reference 1 is short of rendering a true-to-life atmosphere at each of the points. Thus, it is difficult for the listeners to specify a speaker when several people simultaneously speak at each of the points. This results from deterioration of sound separation performance.
Thus, introduction of a multi-channel coding technique, such as the MPEG-AAC technique described in Non-Patent Reference 2, can improve the separation performance. Though providing a true-to-life atmosphere at each of the points, the MPEG-AAC technique overloads the network due to increasing transmission quantity (bit rate). Compared with a speech coding technique, a typical multi-channel coding technique suffers an approximately 100-time increase in bit rate.
Instead of the MPEG-AAC technique, the MPEG-Surround technique described in Non-Patent Reference 3 utilizes a low bit-rate multi-channel coding technique to prevent an increase in bit rate.    Non-Patent Reference 1: ITU-T G.726 standard    Non-Patent Reference 2: MPEG-AAC standard ISO/IEC 13818-3    Non-Patent Reference 3: MPEG-Surround standard ISO/IEC 23003-3    Non-Patent Reference 4: <URL:http://winnie.kuis.kyoto-u.ac.jp/˜ogata/le4-pr/node2.html> searched on the Internet on Jul. 15, 2007    Non-Patent Reference 5: 2000. vol. J83-A. “Scalable Audio Coding Based on Hierarchical Transform Coding Modules.” IEICE The Transactions of the Institute of Electronics, Information and Communication Engineers. A. no. 3 (20000325), pp. 241-252.    Non-Patent Reference 6: <URL:http://www.murata.elec.waseda.ac.jp/˜mura/Research/ICA/ieice99/mld.m> searched on the Internet on Jul. 15, 2007